These days precision metal machining such as drilling, tapping, boring, milling, cutting of inner or outer diameters or surface grinding usually relies on CNC (Computer Numerical Control) machine tools in cooperation with a plurality of cutters or tools. Those cutters or tools mostly have an elongated stem connecting to a driving spindle of the CNC machine tools through a chucking apparatus or chucking structure. The driving spindle rotates in high speed to drive the cutters or tools held by the chucking apparatus to perform machining on a targeted object.
Hence the holding condition of the chucking apparatus on the cutters or tools directly affects the machining precision of the targeted object. If clamping is not tight enough during high speed rotation of the machining tool idle rotation of the cutters or tools occurs. Referring to FIG. 1, a conventional chucking apparatus mainly includes a fastening member 1, a collet 2 and a holding member 3. The holding member 3 is connected to a driving spindle (not shown in the drawing) of a machining tool, and has a housing space 4 to hold the collet 2 to form a tight coupling. The collet 2 is a hollow conical element and has a plurality of conical pawl structures 5 formed in an annular manner with a center passage 6. The pawl structures 5 are elastic. When a cutter or tool is inserted into the collet 2 the inner diameter of the center passage 6 can be adjusted accordingly. The fastening member 1 and the holding member 3 have respectively a corresponding screw thread 7 and 7′ for screwing together to firmly hold the collet 2. In addition, as the collet 2 has the pawl structures 5, when the holding member 3 and the fastening member 1 are screwed together, the pawl structures 5 are forced and shrunk inwards to strengthen coupling of the cutter or tool to increase the firmness of machining and enhance the machining precision of the targeted object.
The collet 2 equipped with the pawl structures 5 to hold the cutter or tool has many advantages, such as can hold cutters or tools of varying sizes and specifications. Moreover, when the holding member 3 and the fastening member 1 are screwed together, they automatically clamp the cutter or tool in an optimal condition. However, the pawl structures 5 can hold only a small portion of cutters or tools. In the event that the cutter or tool is shorter, the pawl structures 5 can provide enough clamping power to keep the cutter or tool from skewing during high speed rotation in the machining process. However, if the cutter or tool is lengthy and the pawl structures 5 can clamp only a distal end thereof, a great portion of the cutter or tool is exposed outside the chucking apparatus. As the targeted object to receive machining usually is a hard article such as metal, and the driving spindle (not shown in the drawing) of the machining tool rotates at the high speed, with a great force applied to the cutter or tool, or the forcing angle being not vertical, the cutter or tool could be skewed. As a result machining precision suffers. It could even cause damage of the tool chucking apparatus or the machining tool and result in a great loss of cost.